Gas detection and measurement system

ABSTRACT

A gas detection and measurement system includes a light source, a light sensor, a test cell body having a first fluid port and a second fluid port, and first and second optical paths from the light source to the light sensor through the test cell. The first and second optical paths have different lengths. As fluid flows through the test cell body, light intensity measurements are taken along the first and second optical paths so that the concentration of a target gas within the fluid can be calculated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 08/658,020, filedJun. 4, 1996, now U.S. Pat. No. 5,770,156.

BACKGROUND OF THE INVENTION

This invention relates to gas detection and measurement, and moreparticularly to an apparatus for measuring gas concentration of aselected gas in a fluid.

Spectrochemical analysis includes a number of techniques for determiningthe presence or concentration of elemental or molecular constituents ina sample through the use of spectrometric measurements. One particulartechnique, spectrophotometric analysis, is a method of chemical analysisbased on the absorption or attenuation of electromagnetic radiation of aspecified wavelength or frequency. A spectrophotometer for providingsuch analysis generally consists of a source of radiation, such as alight bulb; a monochromator containing a prism or grating whichdisperses the light so that only a limited wavelength, or frequencyrange is allowed to irradiate the sample; the sample itself; and adetector, such as a photocell, which measures the amount of lighttransmitted by the sample.

The near ultraviolet spectral region from 200 to 400 nm is commonly usedin chemical analysis. An ultraviolet spectrophotometer usually includesat least a lamp as a radiation source, a sensor and appropriate opticalcomponents. Simple inorganic ions and their complexes as well as organicmolecules can be detected and determined in this spectral region.

In most quantitative analytical work, a calibration or standard curve isprepared by measuring the absorption of a known amount of a knownabsorbing material at the wavelength at which it strongly absorbs. Theabsorbance of the sample is read directly from the measuring circuit ofthe spectrophotometer.

Most gases have at least one well-defined peak of absorption at acertain wavelength. Ozone (O₃), for example, has one peak of absorptionat 253.7 nm, in the ultraviolet range of the spectrum. The concentrationof a selected gas in a sample can be obtained by solving an equation,known as the Beer-Lambert equation as follows:

    I.sub.s =I.sub.r *e.sup.-.di-elect cons.LC

Where:

I_(s) is the intensity of light from the sample;

I_(r) is the intensity of light from the reference;

.di-elect cons. is the ozone absorption coefficient constant at thewavelength used;

L is the length of the absorption chamber (path length of the light);and

C is the concentration of gas in weight/volume.

Since L and .di-elect cons. are fixed quantities, gas concentration canbe determined by measuring the intensities I_(s) and I_(r). TheBeer-Lambert equation provides an absolute determination of gasconcentration. The relationship requires the measurement of a"reference" light intensity and a "sample" light intensity.

Most gas analyzers currently employed for process gas measurement, arefed with gas from a small gas sidestream; and it is the gas from thesidestream that is analyzed. However, the diversion of gas can bewasteful. For example, with respect to ozone generators a portion ozoneoutput from the generator is diverted to the analyzer and subsequentlydirected to a scrubber or gas neutralization/destruction device. Itwould be desirable to eliminate such a diversion and to directly samplemuch of or the entirety of the generator output. This would increase theeffective output of the generator and eliminate the requirement for ascrubber. However, known ultraviolet analyzers are functionally limitedto very small test cells that are unable to accommodate significant flowvolumes.

An additional problem with respect to known light absorption analyzersis that in order to measure the concentration of a given gas, theanalyzer requires a "zero gas present" reference, or a zero reference,to compare with the gas stream. One approach for providing a zeroreference involves using a beam splitter to divert light away from anabsorption cell and an associated sensor for measurement by a secondsensor. However, as the optical components exposed to the gas stream aregradually soiled to varying degrees during usage and the opticalcomponents associated with the diverted light are not soiled, a driftbetween the two measurements develops which becomes progressively moreinaccurate over time.

SUMMARY OF THE INVENTION

The present invention provides a gas detection and measurement systemthat can in many cases allow the entire fluid output of certain gasgenerators to flow through the system while analyzing the fluid todetermine the concentration of a selected gas within the fluid. The gasdetection and measurement system includes a light source, a light sensorarray, a test cell having a first fluid port and a second fluid port,and first and second optical paths from the light source to the lightsensor through the test cell. The first and second optical paths havedifferent lengths. As fluid flows through the test cell, light intensitymeasurements are taken along the first and second optical paths so thatthe concentration of a target gas within the fluid can be calculated.The system minimizes drift problems associated with soiled opticalelements and reduces the frequency of required "zeroing" or calibrationprocedures.

In an exemplary embodiment, the first optical path is defined by a firstpair of spaced-apart optical elements and the second optical path isdefined by a second pair of spaced-apart optical elements. In anotherembodiment of the invention, the first and second optical paths aredefined by a first optical element that is movable with respect to asecond optical element.

DESCRIPTION OF THE DRAWINGS

Other features and benefits of the invention can be more clearlyunderstood with reference to the specification and the accompanyingdrawing in which:

FIG. 1 is a schematic drawing of a gas detection and measurement systemin accordance with the invention;

FIG. 2 is a sectional view of a test cell for the system illustrated inFIG. 1;

FIG. 3 is a cutaway perspective view of the test cell shown in FIG. 2;

FIG. 4 is a schematic illustration of an alternative embodiment of thetest cell; and

FIG. 5 is a schematic illustration of yet another embodiment of the testcell.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a drawing of a gas detection and measurement system inaccordance with the invention. The system includes a light source 10capable of emitting light in a selected spectral range as required forspectrochemical analysis of gas, as is known to those skilled in theart. In the exemplary embodiment the light source 10 emits ultravioletlight useful for analyzing a fluid that includes ozone.

The system further includes a test cell 12 that receives light from thelight source 10 and allows light to exit therefrom. Either a gas or aliquid, hereinafter collectively referred to as a fluid, is directablefrom a first location 14, such as a gas source or generator, through thetest cell 12 to a second location 16, such as a reactor or a gas storageor destruction device. The test cell 12 includes or defines at least oneabsorption chamber or cuvette that provides a first light path having alength L₁ and a second light path having a length L₂ for light passingthrough the test cell 12. Particular values for L₁ and L₂ depend on theoutput of the light source 10, the wavelength employed, sensorsensitivity, and the resolution capability of the system electronics.

A sensor array 18, including one or more sensors 20, is exposed to thelight, if any, that exits the test cell 12 after passing therethrough.When the sensors are on a shared substrate, sensor drift is minimized.The sensor array 18 provides an output indicative of first path lightintensity I_(s1) and an output indicative of second path light intensityI_(s2) to system electronics 22 that include a computation device 24 forperforming required calculations. User defined output based upon themeasurements and calculations is presented on an output device 26 suchas a plotter, a printer, or a video display.

After obtaining measurements of first and second path light intensity,it is possible to determine gas concentration by applying the rewrittenBeer-Lambert equation as follows: ##EQU1## where: I_(r1) is theintensity of light through a reference gas along a first path;

I_(r2) is the intensity of light through a reference gas along a secondpath;

I_(s1) is the intensity of light from a gas sample being evaluated alongthe first path;

I_(s2) is the intensity of light from a gas sample being evaluated alongthe second path;

L₁ is the first path length;

L₂ is the second path length;

ln is a natural logarithm function;

.di-elect cons. is the ozone absorption coefficient constant at thewavelength used; and

C is the concentration of gas in weight/volume.

FIGS. 2-5 illustrate exemplary embodiments of the test cell 12 that areconfigured to provide first and second light paths of unequal lengths.Referring now to FIGS. 2 and 3 collectively, the exemplary test cell 12includes a test cell body 27 that defines a first port 28 and a secondport 30. The ports can be configured to mate with fluid couplings in amanner known to those skilled in the art so that the first port 28 is influid communication with the gas generator 14 and the second port is influid communication with the gas storage device 16. The region betweenthe ports is a cuvette or absorption cell 32 which defines a fluid flowpath for fluid passing through the absorption cell from the first port28 to the second port 30 or from the second port to the first port.

First and second optical paths, 34 and 36 respectively, are providedwhich allow light from the light source 10 to pass through the test cell12, the fluid passing through the absorption cell 32, and to exit fromthe absorption cell and the test cell. In this embodiment, a firstoptical element 38 is aligned with and opposes a second optical element40. Similarly, a third optical element 42 is aligned with and opposes afourth optical element 44. The first optical path 34 is defined by thegap between the first optical element 38 and the second optical element40. The second optical path is defined by the gap between the thirdoptical element 42 and the fourth optical element 44. It should be notedthat the gaps are of different widths and that they define the first andsecond path lengths for use in the above rewritten Beer-Lambertequation. The selection of the particular path lengths is a function ofthe wavelength of light provided by the light source 10, the spectralabsorption characteristics of the fluid, and by the concentration of thegas to be measured. Typically, the gaps or path lengths are not largerthan the diameter of the absorption cell 32. Additionally, although twooptical path lengths are shown, another embodiment of the invention thatis not illustrated includes a third optical path, having no gap, whichprovides additional zero-reference data.

In the illustrated embodiment, the optical elements 38, 40, 42, and 44are optical quality rods that protrude through apertures in a metal orplastic test cell body 27 and into the absorption cell 32 at an angle ofapproximately 90 degrees with respect to the fluid flow path. However,in other embodiments the optical elements are not perpendicular to thefluid flow path and can even be can be parallel to the fluid flow path.The specific angle of orientation of the optical elements, which may liein the range of 0 to 90 degrees with respect to the fluid flow path, isdetermined by particular optical element geometry, size, and desiredflow characteristics within the absorption cell 32.

It has been discovered that a configuration that causes fluid to passthrough the absorption cell 32 in a turbulent manner tends to cause theoptical elements to soil in a substantially uniform fashion. Turbulencecan be induced by extending the optical elements into the fluid flowpath. It should be understood that even though the fluid flow isturbulent, the fluid still moves along a predetermined flow path.Although selected embodiments of the invention call for turbulent flow,the invention is also operable with non-turbulent or laminar flowthrough the absorption cell 32.

An additional consideration with respect to the optical elements is theprevention of cross-talk between the pairs of elements. Therefore, toprevent light from passing from the first optical element 38 into thefourth optical element 44 or from the third optical element 42 into thesecond optical element 40, the pairs of elements are spaced apart adistance that is determined by the diameter of optical elements, thepath lengths, the fluid medium, and the light source. Another approachfor preventing cross-talk is to provide one or more of the opticalelements with shielding 45.

The optical elements can be fabricated from known optical lens materialssuch as fused silica. Additionally, although the optical elementsillustrated herein have circular cross-sections, geometries such assquare, oval, and others are also suitable. Metals and plastics suitablefor fabricating the test cell body 27 include stainless steel, aluminum,PEEK (poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene),VESPEL (poly (5,7-dihydro-1,3,5,7-tetraoxobenzo1,2-c:4,5-c'!dipyrrole-2,6(1H,3H)-diyl)-1,4-phenyleneoxy-1,4-phenylene!),TEFLON (polytetrafluoroethylene) and polyethylene. The particularmaterial selection is determined by the specific application.

The first optical element 38 and the third optical element 42 arepositioned to receive light directly from the light source 10 orindirectly through light guides (not shown) such as optical fibers. Thesecond optical element 40 and the fourth optical element 44 are directlyaligned with the sensor(s) 20 of the sensor array 18. Alternatively,light can be conducted from the second and fourth optical elements, 40and 44, respectively, to the sensor(s) 20 of the sensor array 18 usinglight guides (not shown). The test cell 12 is shown with recesses 46associated with each of the optical elements, wherein the recesses andoptical elements are adapted for mating with light guides. Additionally,the recesses 46 are configured to allow a sealant, such as an O-ring, tobe positioned therein to prevent gas from escaping from the test cellbody 27.

The test cell described with respect to FIGS. 2 and 3 is capable ofbeing dimensioned to permit the full fluid output of the gas generator14 to pass through the absorption cell 32. In an exemplary applicationthe fluid is a gaseous mixture of oxygen and ozone having a maximum flowrate of 15 liters per minute. The absorption cell 32 is approximately0.250 inches in diameter, the optical elements 38, 40, 42, and 44 are0.125 inches in diameter, the first optical path 34 is 0.040 inches inlength, and the second optical path 36 is 0.010 inches in length. Theoptical elements are unshielded and the pairs of optical elements arespaced 0.250 inches apart.

In another embodiment, a test cell having variable geometry featuresprovides more than one optical path length, wherein a first measurementis made at L₁ and a second measurement is made at L₂. Embodiments ofsuch a device are illustrated in FIGS. 4 and 5, wherein optical elementsare movable with respect to each other. With respect to each of thevariable geometry embodiments, it should be noted that the provision ofa single pair of optical elements for making first and second pathlength measurements requires only a single sensor. Additionally, becausethe same pair of optical elements is used to make first and second pathlength measurements, any potential problem associated with differentialsoiling of optical element pairs is eliminated in certain situations.

Referring now to FIG. 4, a test cell 48 is shown which comprises asingle absorption cell 50, and a first optical element 52 that ismovable with respect to an opposing second optical element 54 tolengthen or shorten the distance or path length between the opticalelements. In this embodiment the absorption cell 50 includes a variablegeometry wall portion 56, such as bellows or a resilient material, thatpermits the length of the absorption cell to be changed. The materialselected for the wall portion 56 is determined by the movement requiredand the specific application. In an embodiment of the test cell adaptedfor measuring high concentrations of ozone with ultraviolet light, thedifference between L₁ and L₂ is about 1 mm. A displacement of thismagnitude can be achieved by providing a wall portion 56 fabricated fromspeaker diaphragm material. Turbulent fluid flow can provided within theabsorption cell 50 by offsetting a first port 58 from a second port 60or by providing turbulence generators.

Referring now to FIG. 5, a test cell 62 includes an absorption cell 64defined by fixed length walls includes a first optical element 66 and asecond optical element 68, wherein one or both of the optical elementsare movable from a first position to a second position to provide firstand second path lengths. Although this embodiment is directed to asingle pair of optical elements, a variable geometry embodiment caninclude a second pair of optical elements 70 and 72 that are movablefrom a first position to a second position.

With respect to the embodiments of FIGS. 4 and 5, movement of the wallsor of the optical elements is effected using mechanical and/orelectromechanical devices known to those skilled in the art. Forexample, the rods can be slid back and forth through apertures in a testcell body by electro-mechanical servos.

Although the invention has been shown and described with respect toexemplary embodiments thereof, various other changes, omission andadditions in form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A spectrophotometric flow cell, comprising:a) aflow chamber defining a fluid flow path, having a fluid entrance at oneend of said chamber and a fluid exit at the other end of said chamber;b) a first pair of optical elements, each having an end which projectsinto said chamber, said ends being opposed to one another so as to forma first gap; and c) a second pair of optical elements, each having anend which projects into said chamber, said ends being opposed to oneanother so as to form a second gap greater than said first gap, whereinsaid first and second gaps are in said fluid flow path.
 2. The flow cellof claim 1 wherein at least one of said pairs of optical elements ismovable so as to vary the size of said gap.
 3. The flow cell of claim 1wherein said optical elements are each made of an optically transmissivematerial.
 4. The flow cell of claim 3 wherein said opticallytransmissive material is fused silica.
 5. The flow cell of claim 1wherein one or more of said optical elements have optical shielding onthe surface of one or more said elements in said fluid flow path.
 6. Theflow cell of claim 1 wherein said flow chamber has a circularcross-section.
 7. The flow cell of claim 1 wherein said first and secondpairs of optical elements are parallel to each other.
 8. The flow cellof claim 7 wherein said first pair of optical elements and said secondpair of optical elements are spaced apart by a distance sufficient toprevent cross-talk.
 9. The flow cell of claim 7 wherein said first andsecond pairs of optical elements are transverse to the fluid flow path.10. The flow cell of claim 7 wherein said first and second pairs ofoptical elements are parallel with the fluid flow path.
 11. The flowcell of claim 7 wherein said flow chamber is formed of a materialselected from the group consisting of stainless steel, aluminum, PEEK,VESPEL, fluoropolymers and polyethylene.
 12. The flow cell of claim 1wherein said first and second pairs of optical elements induceturbulence in a fluid flow through said chamber.